Broadcast lowering still hash-repartitions the build and probe sides of a CollectLeft join

[andygrove]

Is your feature request related to a problem or challenge?

When the static distributed planner promotes a join to a broadcast
HashJoinExec(CollectLeft) (maybe_promote_to_broadcast + the broadcast lowering
in plan_query_stages_internal), the join's inputs still carry the join-key
RepartitionExec(Hash) that EnforceDistribution inserted for the original
partitioned join. As a result:

• Build side is hash-repartitioned into a shuffle stage and then written
  again as a broadcast stage — two shuffles where broadcast should need one (just
  replicate the build's natural partitions).
• Probe side still reshuffles on the join key, even though a CollectLeft
  join replicates the build to every probe task and does not require the probe to
  be partitioned on the join key.

So the broadcast promotion adds a broadcast without removing the reshuffles it
was meant to avoid. On TPC-H SF10 (AQE off) the SMJ-broadcast path (#1904) still
gives ~16% because eliminating the sort + collecting a small side helps, but a
large part of the intended benefit — skipping the join-key shuffles entirely — is
left on the table.

This affects both broadcast paths:

• the hash-join broadcast from feat(scheduler): broadcast-style hash join for small-side joins #1647, and
• the sort-merge-join broadcast from feat: broadcast small build side of SortMergeJoinExec in the static planner #1904 (Extend ColocatedJoinRule and BroadcastSmallSideRule to SortMergeJoinExec #1679).

Describe the solution you'd like

During broadcast lowering, strip the redundant join-key RepartitionExec(Hash)
from the converted join's inputs:

• broadcast the build side from its natural (upstream) partitions instead of
  hash-repartitioning then broadcasting;
• keep the probe side at its upstream partitioning rather than reshuffling on the
  join key.

Additional context

Correctness to verify: a CollectLeft join's output partitioning follows the
probe side. Removing the probe's join-key repartition changes the probe (and
therefore the join output) partitioning, so downstream operators that assume
hash-on-join-key partitioning must be re-checked / re-satisfied by
EnforceDistribution/EnforceSorting. Build-side broadcast must still replicate
all build partitions to every probe task.

Follow-up to #1904 (#1679). Related: #342, #348.